Current source

Discussion in 'The Projects Forum' started by Stin, Jul 26, 2012.

  1. Stin

    Thread Starter New Member

    Jul 26, 2012
    8
    0
    Hey guys. Im trying to make a current source from a current source amplifier and a voltage regulator. Ive made it on a bread board as per the app note, but it isnt working as described. Can anyone help me understand how this circuit works? This is the circuit;

    https://docs.google.com/viewer?a=v&...9eeC-H&sig=AHIEtbTnEVLgcCIAa-q1jcqxpFhXHwIoXQ

    Any help would be much appreciated.

    Here are links to the current sense amplifier and the voltage regulator used;

    MAX603
    http://www.maxim-ic.com/datasheet/index.mvp/id/1131

    MAX4072
    http://www.maxim-ic.com/datasheet/index.mvp/id/3387
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Huh, I am stumped as to how this circuit is regulating current. The MAX4072 will measure the current thru R1 (or voltage across) and present a voltage at it's OUT pin.

    The MAX603 is a low dropout regulator that will "buffer" the SET input voltage. So whatever voltage appears at SET will also appear at OUT.

    But HOW the DAC converter plays into this I can't see: I would expect a difference amp to compare the DAC voltage with the MAX4072 voltage and drive the MAX603's SET with that... I would expect the resistors there to be doing that but I can't wrap my brain around how that would work.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    I agree with ErnieM, I don't see how that circuit can work. There appears to be positive feedback from the current sense output to the regulator input. For a current source you need some form of negative feedback, which I don't see. That circuit is a puzzle as to how it's supposed to work. :confused:

    Perhaps an inverting op amp or transistor CE circuit between the current sense output and the voltage regulator input would work. That way as the current output increases the voltage to the regulator would be reduced, as you want for negative feedback. For an inverting op amp, you could apply a positive reference voltage at the positive input to set the current regulation point. When the current sense voltage reaches the reference voltage, the op amp output will start to be reduced, regulating the output voltage (and thus the current). A rail-rail or single-supply type op amp would allow operation from a single supply voltage.
     
  4. Stin

    Thread Starter New Member

    Jul 26, 2012
    8
    0
    Thanks for the replies. I emailed the manufacturer asking for an explanation of how the circuit works, this is the reply.

    The basic principle of
    | this circuit is to control the output current from the MAX603.
    | MAX603 is an LDO which is basically a switch from IN to OUT. This
    | switch is the PFET shown in Figure 2 of the MAX603 datasheet. By
    | adjusting the voltage on SET you can control the Feedback signal
    | (either SET or resistor divider from OUT-R1,21). That feedback
    | signal is than compared to the error amplifier with a 1.2V
    | reference which results in control of the PFET.

    Does this help anyone make sense of the circuit?
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    OK, I see how they get negative feedback. :) The shunt resistor R1 is connected with the - input going to the supply voltage (the small + and - are hard to see on the schematic). That gives an output that sinks output current proportional to the current through R1. This reduces the output voltage provided by the DAC output, thus providing the required negative feedback. That works because MAX4072 is a bidirectional device.
     
    ErnieM likes this.
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